The publication WO 00/04584 discloses an external contact for a semiconductor device which comprises plastic and a metal. In this case, the external contact of the semiconductor device comprises a plastic ball which has an inner body of plastic which is coated on the outside with a metal layer. Such an external contact is preferably used for semiconductor devices in a semiconductor chip format in which the semiconductor device virtually corresponds in its two-dimensional extent to the semiconductor chip. The metal-coated plastic ball achieves the effect of an external contact which is compliant on account of the plastic balls and consequently offers the possibility of connecting the intrinsically rigid semiconductor chip of the semiconductor device in the semiconductor chip format to a higher-level circuit board, such circuit boards of plastic having a much higher coefficient of expansion than the semiconductor device in the semiconductor chip format.
The external contact of the semiconductor device comprising a plastic ball with a metal coating has adequate compliance to compensate for thermal stress under thermal loading between the circuit board and the semiconductor device by virtue of its compliance. The disadvantage of such an external contact is that such external contacts can only be produced with great technical expenditure, since it is first necessary to produce plastic balls and then to coat them with a metal layer. The risk of delamination of such metal coatings cannot be ruled out, so that under thermal loading there is the risk that interruptions in the connection of the circuit board may occur. Consequently, not only is there a high level of production expenditure but also a problem with the reliability of using such compliant external contacts that comprise a plastic core and a metal coating.
In addition, it is known that there is a high demand for external contact balls for the so-called BGA package (Ball Grid Array package). In particular, the demand for lead-free solder balls is great. However, such lead-free solder balls 7, as are shown in FIG. 5, have the disadvantage that intermetallic phases may form, causing such solder balls 7 to become brittle, and consequently a semiconductor device 3 to become sensitive to shock once it has been mounted on a higher-level circuit board 13. Under shock loading, brittle rupture 14 of the external contact 2 may occur on account of the intermetallic phases in the lead-free solder balls 7. Furthermore, there is a demand to push ahead with the miniaturization of such lead-free solder balls 7, so that the three-dimensional extent is further reduced.
In order nevertheless to increase the shock immunity, an insulating and stabilizing underfill material is introduced between the surface-mountable solder balls of the BGA package and the higher-level circuit board, with the intention of reducing the shock sensitivity of the solder balls with intermetallic phases. For this purpose, however, a reserve area has to be provided on the higher-level circuit board around the semiconductor device with lead-free solder balls, in order to introduce this underfill material. Furthermore, the introduction of such an underfill material means additional expenditure for the customer wishing to load a higher-level circuit board.